Method of forming a composite tubular object



METHOD OF FORMING A COMPOSITE TUBULAR OBJECT "iled June 20, 1958 F. L. BROWN July 3, 1962 2 Sheets-Sheet 2 mmmdkm .C 23

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Unitcd States Patent Bfi lljll? Patented July 3, 1952 3,041,717 METHOD OF FORMING A COMPOSITE TUBULAR OBJECT Fred L. Brown, Chicago, 111., assignor to Acme Steel Company, Chicago, 111., a corporation of Illinois Filed June 20, 1958, Ser. No. 743,300 14 Claims. (Cl. 29-446) This application relates to the art of deep drawn tubular or cup shapes from metal sheets and particularly to improvements in composite deep drawn tubular or cup shapes drawn from multiple layers of metal sheets.

With the production of tubular or cup shaped metal objects, it is often desirable to provide such objects with layers of different metals which metals each serve separate functions. For example, when producing flashlight battery cases, it is desirable to provide an inner casing or shell of zinc for electrolytic purposes and an outer shell of steel for strength and durability and as a protector shield to prevent outward leakage of electrolyte in case the zinc corrodes through by action of the electrolyte. In other cases, cup shaped metal containers or drums are used for shipment of precision instruments or other type fragile or delicate equipment parts in order to provide adequate protection. The drums are ordinarily coated with durable painted finishes, but it has been suggested that steel drums coated with aluminum or brass would be desirable. In another case, it is common to provide electrolytic capacitors in aluminum cans, the aluminum providing the proper electrical characteristics. Again it has been suggested that a strong casing of steel coated with a thin coating of aluminum would be desirable.

In all cases mentioned, it is desirable to provide an inexpensive method of manufacturing a composite metal tube or cup of two or more metal layers, one of which is preferably a relatively inexpensive metal such as steel. By the use of a cheaper metal such as steel, it is possible to minimize the amount of more expensive metal, such as aluminum or brass, used and thereby reduce the overall cost, provided the method of manufacture is sufficiently economical.

Various futile attempts have been made to perform the task sufliciently economically to be commercially practical. In the case of battery shells, a relatively thick coating of zinc is required and no practical inexpensive coating method has been found. As a result, separate cups of steel and zinc are in use, but still with some sacrifice in economy and other disadvantages over a battery using just a zinc casing alone. In the other situations mentioned, it is also apparently commercially impractical because of the expense involved to provide good composite cups. Again, in the case of battery shells, it is important that the inner and outer cups composing the composite shell are in extremely close contact with each other. The close contact is an important factor in prolonging the battery life. If any space is provided between the inner cup of zinc and the outer cup of steel, when the zinc is corroded through by the acid at only a pin hole, the acid can leak through the pin hole and into the space and contact a larger surface of the outside wall of the zinc cup to accelerate corrosion and thereby decrease battery life.

It is an object of the invention to provide an improved deep drawn tube or cup composed of a plurality of tubes or cups, one within the other, which are held tightly bonded together in close intimate relationship with each other without spaces between the mating cup wall surfaces.

It is another object of this invention to provide an improved method for forming the improved deep drawn tubes or cups from multiple layers of metal sheets whereby the multiple layers of metal are drawn simultaneously into tubular or cup shapes with intimate contact between mating cup wall surfaces.

It is another object of the invention to provide an improved deep drawn tube or cup composed of a plurality of tubes or cups which are formed together in a simple and economically practical manner.

In order to accomplish these objects, the two pieces of sheet metal which are to be formed into the composite cup are positioned one upon the other and simultaneously deep drawn uniformly in one or more steps as required in order to produce the tubular or cup shape. As the two metal sheets are deep drawn, they are plastically worked and stressed to values exceeding their yield points. When the deep drawing means are released from the freshly formed tube or cup, the forces causing the deformation of the metal sheets are thereby released and the natural spring back of the two metal pieces causes partial spring return of the two metal pieces. It has been found that, by having the two metal sheets characterized primarily as having different spring back characteristics, it is possible to have one tube or cup spring back more than the other with the result that the two tubes or cups forming the composite parts of the final tubular or cup shape engage each other at their abutting surfaces in very close intimate contact. The effect appears to be that the tube or cup tending to spring back the most is prevented from doing so by the tube or cup which does not tend to spring back as much and a condition of interference of one tube or cup with the other tends to exist. Because the residual stresses seem to be high, the proximity of the abutting surfaces of the tubes or cups is very close and the appearance of the tubes and cups is almost as if one layer of metal is plated upon the other. The invention can be better understood with reference to the accompanying drawings in which FIG. 1 shows a cup shaped casing or cylinder composed of two cups, one within the other, as it would appear after being produced by the method of this invention;

FIG. 2 shows an edge view of two superimposed metal sheets in readiness to be formed into a cup shaped object as shown in FIG. 1 by means of punch and die members shown in phantom outline;

FIG. 3 shows the two superimposed metal sheets of FIG. 2 after they have been formed into a cup shape by means of the punch and die members;

FIG. 4 shows a diagram indicating the stress-strain and spring back characteristics of two metal sheets used to form a composite cup;

FIG. 5 shows a similar diagram to FIG. 4 except that certain relative conditions of the metals are different;

FIG. 6 shows actual stress-strain curves for several different metals which are capable of use with this invention;

FIG. 7 shows an additional stress-strain diagram for a situation where two identical metals are used for both inner and outer cups; and

FIG. 8 shows another stress-strain diagram for still another relationship.

As viewed in FIG. 1, a preferred product of this invention comprises a composite cup 1 which consists of an inner cup 2 and an outer cup 3. The outer wall of the inner cup 2 is in close and intimate contact with the inner wall of the outer cup 3 in a region 4. The cup 1 shown is of a size and shape adapted for use as a flashlight dry cell battery casing. With the additions of the ordinary electrolyte, center electrode and cover, the completed battery can easily be manufactured. The outer cup 3 is preferably composed of steel to provide strength for the 0 casing and to act as a seal to block the flow of fluid from escaping through openings which, through use, may be corroded through the inner cup 2 which is preferably composed of zinc as in the ordinary flashlight battery. The reason why it is desirable and most advantageous to have the two cups 2 and 3 in very close intimate contact is to. prolong the useful life of the battery. This can be understood by considering that the zinc cup 2, by action of the acid within the electrolyte may be eaten or corroded away to the point where a pin hole, for example, appears entirely through the wall of the zinc cup 2. When this occurs, the acid of the electrolyte is free to pass out of the pin hole into the region of the steel cup 3. If the two cups 2 and 3 are provided with a relatively loose fit so that spaces appear between them, even though very thin, as the acid leaks through the pin hole, it is free to fill up the space and contact a larger surface of the outside wall of the zinc cup 2. When this occurs, the acid corrosion travels at a much faster rate because it contacts the larger surface with the effect that the useful life of the battery is considerably shortened. With the product of this invention the two cups 2 and 3 are in very close contact with each other and, if a pin hole does penetrate through the wall of the zinc cup 2, there being no space between the cups into which the pin hole leads, the acid can only act on the zinc in the region of the pin hole rather than overflow the outside surface of the zinc cup. In this manner, the acid corrosion is retarded to the point that the effective useful life of the dry cell is extended.

In order to produce the cup 1 of FIG. 1, two metal sheets 5 and 6 are positioned with one sheet 5 superimposed upon the other sheet 6. The sheets 5 and 6 are positioned centrally between punch 7 and die 8 into which the punch 7 can be fed. In order to form the sheets 5 and 6 into a cup 1, the punch 7 is lowered to draw the sheets 5 and 6 simultaneously into a cup shape as shown in FIG. 3. The punch 7 and the die 8 are shown in phantom lines and deliberately shown with spaces between the cup portions and the punch and die portions only for the sake of clarity of illustration. Actually, it is apparent that the punch and die members 7 and 8 would and should be in very close contact with the cup portions being formed. Ordinary clamping means 8d which is commonly referred to as a pressure plate, and usually in the form of a spring loaded ring, is provided to clamp the metal sheets 5 and 6 against the die 8 and prevent the formation of wrinkles at margins of the sheets due to the circumferential compressive stresses created during the drawing operation. After the punch and die members 7 and 8 have formed the metal sheets into a cup 1, the metal sheet 5 becomes the inner cup 2 while the metal sheet 6 becomes the outer cup 3.

Up to this point, it has not been indicated in detail as to how it is insured that there will be intimate contact between the cups 2 and 3. Broadly speaking, this is accomplished, as already stated, by choosing the metals such that their amounts of spring back will be different from each other to result in a tendency toward interference of the metal of one cup with that of the other such that a natural tight bond is achieved.

The stress-strain diagram of FIG. 4 shows the stressstrain curves for two different metals. The vertical axis represents stress per unit cross section while the horizontal axis represents unit strain or elongation. Curve 9 shows the stress-strain relationship of one particular metal to be used for the outer cup 3 which is made from the fiat sheet 6. Curve 10 shows the stress-strain relationship for the metal sheet 5 used for the inner cup 2. It should be noted that the curve 9 may be divided into two portions 11 and 13 and curve 10 can be divided into two portions 14 and 15. The portions 11 and 14 of the curves represent the regions up to the elastic limits of the metals and the elastic limits are represented by points 16 and 17. The portions 13 and are the remaining portions of the curves where the metal plastically deforms permanently.

In the formation of the composite cup 1 from the sheets 5 and 6, it is assumed that the outer cup is deformed more than the inner cup because the metal in the outer cup must be stretched further. The total amount of strain for the inner cup is shown by line 18 while that of the outer cup by 18a. Upon release of the punch and die members 7 and 8 from the metal sheets 5 and 6 after they are formed into the cup 1, there is a natural spring back in both pieces of metal and it is known that the spring back follows a relationship having the same slopes as the original portions 11 and 14 which represent the state of the metals in the elastic regions. In so doing, the spring back lines 19 and 20 are then represented as having the same slopes as 111 and 14, respectively. After full release of the punch and die members 7 and 8, the inner cup 2 is theoretically returned by spring back by an amount equal to 21. Similarly, the theoretical spring back of cup 3 is represented by 22. Therefore, since the outer cup 2 theoretically springs back more than the inner cup 3, there is an amount of apparent interference between the two cups 2 and 3 which is equal to the difference between 22 and 21. But, since the cups react against each other, they are actually prevented from overlappingly interfering with each other, with the result that they are both left with residual stresses which force the abutting surfaces of the cups 2 and 3 tightly against each other. The cups both tend to spring back their theoretical amounts, but are prevented from doing so by the effect of one on the other. With this arrangement, by the proper choice of metals, it is possible to have a composite cup composed of a plurality of cups which are in very close contact with each other. Their proximity can be so intimate that it can approximate the condition of one metal being plated upon the other.

The stress-strain diagram of FIG. 5 shows the relationship of two metals used for the cups 2 and 3 of the cup 1 except that the metals are characterized by having a greater slope in the elastic range for the inner cup than for the outer cup. This diagram shows the slope of the elastic region 24 of the curve 25 for the metal used in the outer cup as greater than the slope of the elastic portion 26 of the curve 27 of the metal used for the inner cup. With this diagram, it is apparent that, after release of stress, the total final theoretical spring back 28 of the inner cup is less than the spring back 29 of the outer cup. This leaves a theoretical overlap of spring back or strain equal to the difference between .28 and 29'. This theoretical overlap does not actually occur. Instead, it causes the cups to press tightly as they tend toward a theoretical overlap or interference. So, similarly, as in the case described for the diagram of FIG. 4, by use of metals having the characteristics of the diagram of FIG. 5, it is possible to provide a composite cup composed of inner and outer cups in close intimate contact with each other.

Although the invention so far has been described for only two layers of metal, the principles are equally applicable to more than two layers of metal and it is possible to use more than two layers and still obtain close contact between the resulting cup shapes.

Also, although the invention has been described for only a circular cylindrical shape, it can be applied equally well to other cylindrical shapes such as of square, triangular or other conventional cross sectional shapes. Also, instead of forming cup shapes, hollow open ended tubes can be formed by the same principle by deep drawing multiple sheets of metal which are provided before hand with centrally located apertures. The apertures or openings can be of the same shape as the final cross sectional tube shape desired. For example, if a circular tube is desired, then the sheets might be provided with a circular opening. If the tube to be formed is to be of square cross section, then square holes would seem appropriate. The reason for making the holes of the same shape as the cross sectional shape of the tube to be formed is to minimize the amount of trimming required after forming.

Even if holes are not provided in the sheets prior to forming, open ended tubes can be provided after a cup is formed by merely removing the closed end of the cup.

For specifically producing a dry cell battery case, a sheet of .007 inch thick zinc was superimposed upon a sheet of .017 inch thick steel, both sheets measuring approximately 3% inch diameter. The two sheets were drawn simultaneously into cup shapes in six stages with appropriately dimensioned ordinary type punch and die means. The punch and die were ground smooth and uniform so that they operated to draw the metal sheets into close uniform contact with each other throughout the entire sheet surfaces. This is necessary in order to guarantee uniform spring back so that the formed cups have uniform contact after release of the drawing means. The following table shows the dimensions of the composite cup after the successive drawing operations.

Other different combinations of metals also can be used to form composite cups. Examples of materials which work satisfactorily are:

Example Outside Cup Inside Cup .0075 zine. .009 steel. .005 brass- .010" steel.

.009 steel 005" brass. .010" steel. 0075 zine. .009" steel .004 aluminum .009 steel .009" steel. 1

H .004" a uminum 7 sted n {.004 aluminum,

1 Two sheets used for inside cup.

In these seven examples, the initial sheets of metal were circular blanks of 1% inch diameter. The blanks were drawn in one or more steps into cup shapes having 1%; inch outside diameter and a depth of inch.

The metals in each case were of the following types:

Modulus Yield Common Name Composition of Elas- Point,

ticity p.s.i.

Cold Rolled Steel 007-0129} C-.. 30, 000, 000 38, 000 Yellow Brass (Half Hard)" GS'Z /CYr- 15,000,000 40,000

Deep Draw Zinc 08% Pb 13, 020, 000 12, 500 Cold Rolled Aluminum No. 1145, 99.0+% Pure 10, 000, 000 6, 000

Aluminum 00. of America.

Considering Examples 2 and 3, it appears that there may be some inconsistency because in one example the brass is on the outside of the steel while in the other case it is on the inside of the steel. It might seem that in one case it should work and in the other case it should not. But, if it is remembered that the outer cup is deformed or worked more than the inner cup, it is apparent that the result can be achieved by a study of both FIGS. 4 and 5. Note that the basic similarity between FIGS. 4 and 5 is that the outer cup is shown to be strained more than the inner cup, while the basic difference is that the inner cup is represented by the upper curve in FIG. 4 and by the lower curve in FIG. 5. If it is assumed, for example, that the steel is represented by the upper curve in both FIGS. 4 and 5 because the stress-strain curve for steel is actually higher in all cases than that of brass, then it is obvious that for both figures, a state of theoretical interference between the inner and outer cups is present which manifests itself as a binding between the cups, as already described.

Considering Example 6, it also may seem to be inconsistent since steel is used in both the inner and outer cups. However, with reference to FIG. 7, it can be shown that there is no inconsistency. Assume that the inner and outer cups are both drawn from identical steel sheets insofar as their stress-strain relationship is concerned. If so, then their stress-strain curves are identical and would appear as a single curve 40. The inner cup being stressed less, would be strained to point 41 on the curve while the outer cup, being stressed more, would be strained to point 42 on the curve. Since the slope of even the plastic region 43 of the curve 40 is angularly upward, upon spring back of the two cups after release of the drawing stresses, it can be shown that the outer cup springs back more than the inner cup, with the result that the two cups grip each other tightly in a tight bond. If triangles are formed between the points 41, 42, the two perpendiculars 44, 45 to the abscissa, and the spring back lines 46, 47 as shown because the point 42 is higher than the point 41 on account of the upward slope of curve 43, the triangle 42, 44, 46 is larger than the triangle 41, 45, 47. Consequently, it follows that the actual spring back 48 of the outer cup is greater than the spring back 49 of the inner cup and the difference between them is the amount of theoretical interference which causes the binding between the cups.

Considering Example 7 which has an outer cup of steel and two inner cups of aluminum, the theory as applied to FIG. 7 can be related to the two aluminum cups while that of FIG. 5 can be applied to the steel cup and the middle cup of aluminum.

With reference to FIG. 8, a situation is shown where, unlike the curves of FIGS. 4 and 5, the upper curve 50 having the greater spring back curve slope is used to represent the characteristics of the metal of the outer cup, and the lower curve 51 having a less spring back curve slope is used to represent the characteristics of the metal of the inner cup. In such a case, the actual spring back amounts 52 and 53 are different, but that of the inner cup is greater than that of the outer cup. The result is a loose fit between the cups because there is no tendency toward interference between the inner and outer cups. This is the situation which is avoided by practice of this invention.

It is therefore known, because of what has been described, that it can be determined graphically whether or not two or more metal sheets will form cups which bind after being drawn simultaneously into cup shape forms. From a more practical point of view, the result also can be achieved by trial and error, But, even in that case, any conclusions reached from a graphic analysis can be applied so as to guide the trial and error tests. In any case, by trial and error, if proper results are not achieved, the theories advanced by the teachings of this invention can be applied in order to understand why the proper results are not attained and proper changes in material can be made accordingly. It should be emphasized from all that has been described that three factors seem to determine whether or not the proper binding effect can be achieved between the mating metal cups. They are 1) the relative heights of the strain-straincurves, (2) the relative slopes of the stress-strain curves, and (3) the relative amounts of stress and strain between the metals indicated by the relative distances reached along the stress-strain curves during the drawing operations.

So far, the thicknesses of the metals used have not been considered. However, it is believed that the thicknesses of the metals do affect the results by varying the third factor mentioned. Since the stress-strain diagrams show the curves along the neutral axes of the metals being deformed, it is apparent that the thinner the metals used for the cups, the less difference there will be between the amounts of strain in each cup. But, if the metals are thick, the neutral axis of the outer cup will be deformed through a much longer deformation path than the neutral axis of the inner cup with the result that the outer cup will be strained considerably more than the inner cup, thus possibly being the factor determining whether or not the inner and outer cups will bind upon release of the drawing stresses.

By referring to FIG. 6, the stress-strain curves for the different metals shown are all different and it is apparent that many different combinations of metals can be made to work in the practice of this invention. Note that just as identical metals such as identical steels can be used successfully as described, the metals can be different. For example, it is logical that by proper adjustment of the three factors mentioned that hard brass can be used with annealed brass or low carbon cold rolled steel can be used with stainless steel, and whether one metal must be used as the inner or outer cup can be determined based upon the principles taught by this invention.

Although the invention has been described in somewhat particular terms, it should be understood that the invention can be varied considerably without departing from the true scope of the invention.

I claim:

1. A method of producing from a plurality of metal sheets a composite cup composed of a plurality of cups one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into cup shapes one within the other, the cups being stressed during the drawing operation to strain the outer cup more than the inner cup, the physical properties of the metals of the sheets forming the cups being such that, upon release of the drawing stresses, the amounts of natural spring back of the cups are sufiiciently different that the cups grip each other with intimate contact between their adjacent mating surfaces.

2. A method of producing from a plurality of metal sheets a composite cup composed of a plurality of cups one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into cup shapes one within the other, the cups being stressed during the drawing operation to strain the outer cup more than the inner cup, the physical properties of the metals of the sheets forming the cups being such that, upon release of the drawing stresses, an outer cup tends to spring back more than the next inner adjacent cup to result in these cups gripping each other with intimate contact between their adjacent mating surfaces.

3. A method of producing from a plurality of metal sheets a composite tubular object composed of a plurality of tubular objects one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into tubes one within the other, the tubes being stressed during the drawing operation to strain the outer cup more than the inner cup, the stress-strain characteristics of the metals of the sheets forming the tubes being such that, upon release of the drawing stresses, the amounts of natural spring back of the tubes are sufficiently different that the tubes grip each other with intimate contact between their adjacent mating surfaces.

4. A method of producing from a plurality of metal sheets a composite tubular object composed of a plurality of tubular objects one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into tubes one within the other, the tubes being stressed during the drawing operation to strain the outer tubes more than the inner tube, the physical properties of the metals of the sheets forming the tubes being such that, upon release of the drawing stresses, an outer tube tends to spring back more than the next inner adjacent tube to result in these tubes gripping each other with intimate contact between their adjacent mating surfaces.

5. A method of producing from a plurality of metal sheets a composite open ended tube composed of a plurality of tubes one within the other, comprising superimposing metal sheets upon each other and drawing the metal sheets simultaneously into cup shapes nested one within the other, the cups being stressed during the drawing operation to strain the outer cup more than the inner cup, the stress-strain characteristics of the metals of the sheets forming the cups being such that, upon release of the drawing stresses, an outer cup tends to spring back more than the next inner adjacent cup to result in these cups gripping each other with intimate contact between their adjacent mating surfaces, and then shearing the bottom closed portion from the nested cups to thereby provide a composite open ended tube.

6. A method of producing a battery case from a plurality of metal sheets comprising superimposing a plurality of metal sheets upon each other and drawing the metal sheets simultaneously into nested cup shapes one within the other, the cups being stressed during the drawing operation to strain the outer cup more than the inner cup the stress-strain characteristics of the metals of the sheets forming the cups being such that, upon release of the drawing stresses, an outer cup tends to spring back more than the next inner adjacent cup to result in these cups gripping each other with intimate contact between their adjacent mating surfaces.

7. A method of producing a battery case comprising superimposing a zinc sheet upon a steel sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the zinc sheet is formed within the steel sheet, the cups being stressed during the drawing operation to strain the outer steel sheet more than the inner steel sheet, and then releasing the externally applied stress to permit spring back of both the then formed zinc and steel cups, the physical properties of the metals of the sheets being such that the outer steel cup tends to spring back more than the inner zinc cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

8. A method of producing a composite cup comprising superimposing a brass sheet upon a steel sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the brass sheet is formed within the steel sheet, the cups being stressed during the drawing operation to strain the outer steel sheet more than the inner brass sheet, and then releasing the externally applied stress to permit spring back of both the then formed brass and steel cups, the metal sheets having such physical characteristics that the outer steel cup tends to spring back more than the inner brass cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

9. A method of producing a composite cup comprising superimposing a steel sheet upon a brass sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the steel sheet is formed within the brass sheet, the cups being stressed during the drawing operation to strain the outer brass sheet more than the inner steel sheet, and then releasing the externally applied stress to permit spring back of both the then formed steel and bras cups, the metal sheets having such physical characteristics that the outer brass cup tends to spring back more than the inner steel cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

10. A method of producing a composite cup comprising superimposing an aluminum sheet upon a steel sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the aluminum sheet is formed within the steel sheet, the cups being sressed during the drawing operation to strain the outer steel sheet more than the inner aluminum sheet, and then releasing the externally applied stress to permit spring back of both the then formed aluminum and steel cups, metal sheets having such physical characteristics that the outer steel cup tends to spring back more than the inner aluminum cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

11. A method of producing a composite cup comprising superimposing a steel sheet upon a steel sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the steel sheet is formed within the steel sheet, the cups being stressed during the drawing operation to strain the outer steel sheet more than the inner steel sheet, and then releasing the externally applied stress to permit spring back of both the then formed steel cups, the physical characteristics of the metal sheets being such that the outer steel cup tends to spring back more than the inner steel cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

12. A method of producing a composite cup comprising superimposing a zinc sheet upon a steel sheet and applying an external stress to draw the two sheets simultaneously into nested cup shapes whereby the zinc sheet is formed within the steel sheet, the cups being stressed during the drawing operation to strain the outer steel sheet more than the inner zinc sheet, and then releasing the externally applied stress to permit spring back of both the then formed Zinc and steel cups, the physical characteristics of the metal sheets being such that the outer steel cup tends to spring back more than the inner zinc cup to result in the two cups gripping each other with intimate contact between their adjacent mating surfaces.

13. A method of producing from a plurality of metal sheets a composite cup composed of a plurality of cups one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into cup shapes one within the other, the metals being stressed during the drawing operation to strain the cups into uniform and close contact with each other, the outer cup being strained more than the next adajacent inner cup, the stress-strain characteristics of the metals of 10 the sheets forming the cups being such that, upon release of the drawing stresses, the amounts of natural spring back of the cups are sufficiently diiferent that the cups grip each other uniformly with a greater force between their adjacent mating surfaces than before the release of the drawing stresses.

14. A method of producing from a plurality of metal sheets a composite tube composed of a plurality of tubes telescoped one within the other, comprising superimposing one metal sheet upon another and drawing the metal sheets simultaneously into tubular shapes one telescoped within the other, the metals being stressed during the drawing operation to strain the tubes being formed into uniform and close contact with each other, the outer tube being strained more than the next adjacent inner tube, the stress-strain characteristics of the metals of the sheets forming the tubes being such that, upon release of the drawing stresses, the amounts of natural spring back of the tubes are sufliciently ditferent that the tubes grip each other uniformly with a greater force between their adjacent mating surfaces than before the release of the drawing stresses.

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